Crotylboronates achiral

Solutions to problems (i) and (ii) were already available as a result of studies by Hoffmann and Wuts on the reactions of 7-alkoxyaIlyl-boronates with achiral aldehydes (Figure 6). 3 Relatively little information was available, however, regarding the stereochemistry of such reactions with chiral aldehydes. Hoffman had published several examples of reactions of (E)- and (Z)-crotylboronates (methyl replacing OMe in Figure 6) with chiral aldehydes such as 2-methylbutanal, but the best diastereofacial selectivity that had been reported was only 83 17.14 Thus, it was by no means certain that the chemistry summarized in Figure 7 would be successful.3a... 

Handling, Storage, and Precautions typically the reagents are handled as solutions in toluene (0.5-lM) and transferred by syringe under an inert atmosphere stored neat or as a solution in toluene over 4 A molecular sieves under an argon atmosphere in a refrigerator (—20 °C), the reagent is stable for many months. In the presence of water, (1) rapidly hydrolyzes to achiral crotylboronic acid, the presence of which leads to reduced enantioselectivity in reactions with aldehydes. 

Roush, W. R., Ando, K., Powers, D. B., Halterman, R. L., Palkowitz, A. D. Enantioselective synthesis using diisopropyl tartrate-modified (E)-and (Z)-crotylboronates reactions with achiral aldehydes. Tetrahedron Lett. 1988, 29, 5579-5582. 

In reactions of a-methyl chiral aldehydes with achiral (Z)-crotylboronates, the anti-Felkin adduct (cf. 107b) is favored (for further discussion see Section 11.2) [3, 65]. In the double asymmetric reaction of 97b and (S,S)-213, the anti,syn-di-propionate 107b is obtained with high selectivity (selectivity=95 5). The stereochemistry of 107b is consistent with product formation via the matched anti-Felkin transition state 247. Finally, the, vyn,5y -dipropionate 106c is obtained as the major product from the mismatched reaction of the TBDPS-protected aldehyde 97c with (f ,R)-(Z)-213 this reaction, however, is not sufficiently stereoselective to be synthetically useful (selectivity = 64 36). The mismatched transition state... 

Results of representative reactions of substituted allylboronates and achiral aldehydes are summarized in Table 1. It is noteworthy that in the majority of cases the reaction diastereoselectivity closely parallels the isomeric purity of the reagents, thus underscoring the requirement that the allylboronate syndesis be highly stereoselective. Dimethyl crotylboronates (68) and (69) are more reactive than (1) and (2), as indicated by the fact that the reactions of (68) and (69) are complete within a few hours at -78 °C while those of (1) and (2) include an overnight period at room temperature. Reagents (73)-(79) are even less reactive than (1) and (2), their reactions requiring several days at room temperature to reach completion. A detailed study of the temperature dependence of diastereoselectivity, however, has not been reported to date. 

Yamamoto and coworkers have studied the reactions of crotyl-9-BBN (47) and achiral aldimines (Scheme 15). These reactions occur at much lower temperature than those involving crotylboronates because of the greater reactivity of (47). No clear stereochemical pattern, however, is apparent in the data. Assuming that (47) reacts preferentially as the ( )-crotyl isomer, one would expect anti dia-stereomer (93) to be the major product via transition state (95a Scheme 16) by analogy to Hoffmann s results with oximes and oxime ethers (Schemes 8 and 14). Only in entries 1, 6, 7 and 8 of Scheme 15, however, is this stereochemical result realized. Yamamoto argues that syn diastereomer (92), the major... 

Excellent double diastereoselection has also been realized in the reactions of (151) and chiral crotyl-boron reagents (Table 7). Interestingly, the best selectivity for diastereomers (153) and (156) is obtained by using the tartrate crotylboronates (S,S)-(18) and () , )-(19), respectively (entries 2 and 3), while Masamune s 2,5-dimethylborolane reagents (HJl)-(221) and (5,5)-(222) provide the greatest selectivity for diastereomers (152) and (157 entries 7 and 10). Comparative data for the diastereoselectivity ol tained with the achiral crotylteronates (1) and (2) appear in the last two entries of Table 7. 

This reaction was first reported by Roush in 1985. It is an enantioselective synthesis of a chiral alcohol by the condensation between a crotylboronate derived from diisopropyl-(/ ,/ )-tartrate and an achiral aldehyde. Therefore, it is known as the Roush crotylboration, ... 

The synthesis and use of tartrate-modified allylboronate 1 was first reported by Roush and co-workers in 1985. The synthesis and use of the corresponding (E)- and (. -crotylboronate reagents 2 and 3 was published by Roush and co-workers shortly thereafter. The ease of synthesis, stability and efficient reactivity of these reagents offers advantages over many other allyl- and crotylmetal reagents. Roush and co-workers have extensively explored the enantioselective allylations with achiral aldehydes as well as the... 

The reactions of reagents 1-3 with a variety of achiral aldehydes have been reported by Roush and co-workers. A few of these reactions are summarized in Table 1. The diastereoselectivity is uniformally high (> 95 5) while the enantioselectivity varies from moderate to excellent (55-91% ee), depending upon the aldehyde and reagent structure. The (jE)-crotylboronate 2 is generally the most enantioselective while the (. -crotylboronate 3 is... 

Roush WR. Ando K, Powers DB, Palkowitz AD, Halterman RL. Asymmetric synthesis using diisopropyl tartrate modi-Hed (E)- and (Z)-crotylboronates preparation of the chiral crotylboronates and reactions with achiral aldehydes. J. Am. Chem.Soc. 1990 112 6339-6348. 

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